High-energy electrolytic capacitors for implantable defibrillators

ABSTRACT

Implantable defibrillators are implanted into the chests of patients prone to suffering ventricular fibrillation, a potentially fatal heart condition. A critical component in these devices is an aluminum electrolytic capacitors, which stores and delivers one or more life-saving bursts of electric charge to a fibrillating heart. To reduce the size of these devices, capacitor manufacturers have developed special aluminum foils, for example core-etched and tunnel-etched aluminum foils. Unfortunately, core-etched foils don&#39;t work well in multiple-anode capacitors, and tunnel-etched foils are quite brittle and tend to break when making some common types of capacitors. Accordingly, the inventors devised a new foil structure having one or more perforations and one or more cavities with a depth less than the foil thickness. In an exemplary embodiment, each perforation and cavity has a cross-sectional area, with the perforations having a larger, for example, 2 to 100 times larger, average cross-sectional area than the cavities. Other embodiments of the invention include foil assemblies, capacitors, and implantable defibrillators that benefit from properties of the new foil structure.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.09/165,779, filed on Oct. 2, 1998, now issued as U.S. Pat. No.6,556,863, the specification of which is incorporated herein byreference.

This application is also related to U.S. patent application Ser. No.09/606,633, filed on Jun. 29, 2000, now issued as U.S. Pat. No.6,421,226 and U.S. patent application Ser. No; 09/606,291, filed on Jun.29, 2000, now issued as U.S. Pat. No. 6,426,864, the specifications ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention concerns electrolytic capacitors, particularlythose for use in medical devices, such as implantable defibrillators.

Every year more than half a million people in the United States sufferfrom heart attacks, more precisely cardiac arrests. Many of thesecardiac arrests stem from the heart chaotically twitching, orfibrillating, and thus failing to rhythmically expand and contract asnecessary to pump blood. Fibrillation can cause complete loss of cardiacfunction and death within minutes. To restore normal heart contractionand expansion, paramedics and other medical workers use a device, calleda defibrillator, to electrically shock a fibrillating heart.

Since the early 1980s, thousands of patients prone to fibrillationepisodes have had miniature defibrillators implanted in their bodies,typically in the left breast region above the heart. These implantabledefibrillators detect onset of fibrillation and automatically shock theheart, restoring normal heart function without human intervention. Atypical implantable defibrillator includes a set of electrical leads,which extend from a sealed housing into the heart of a patient afterimplantation. Within the housing are a battery for supplying power,heart-monitoring circuitry for detecting fibrillation, and a capacitorfor storing and delivering a burst of electric charge through the leadsto the heart.

The capacitor is typically an aluminum electrolytic capacitor, whichusually includes a sandwich-like assembly of two strips of aluminum foilwith two strips of paper, known as separators, between them. One of thealuminum foils serves as a cathode (negative) foil, and the other servesas an anode (positive) foil. Sometimes, two foils are stacked one on theother to form a dual anode. Attached to each foil is an aluminum tabwhich electrically connects the foil to other parts of the capacitor.

The foil-and-paper assembly, known as an active element, is then placedin a case, usually made of aluminum, and the paper is soaked, orimpregnated, with a liquid electrolyte—a very electrically conductivesolution containing free positive or negative ions. After the paper isimpregnated, the case is sealed shut with a lid called a header.Extending from the header are two terminals connected respectively tothe anode foil and cathode foil via the aluminum tabs.

In recent years, manufacturers of aluminum electrolytic capacitors haveimproved capacitor performance through the development of aluminum foilswith increased surface areas. Increasing surface area of a foil,particularly the anode foil, increases capacitance and thus thecharge-storage capacity of a capacitor.

One approach to increasing surface area of a foil is to chemically etchmicroscopic hills and valleys into both sides of the foil. The etchingdepth is controlled to leave a solid core layer between the sides of thefoil. Thus, foils with this type of etching are called “core etched.”Although core-etched foils have more surface area, they don't work aswell as expected in capacitors with two stacked anode foils, because thesolid core layer of one anode foil shields the other anode foil fromelectrolyte flow.

Another approach, known as tunnel etching, entails etching both sides ofa foil to form millions of tiny holes, or tunnels, completely throughthe foil, from one side to the other. The tunnels, which typically havean approximately circular cross-section about one-micron in diameter,allows electrolyte to flow through the foil. Thus, tunnel-etched foilsovercome the electrolyte-flow problem of core-etched foils.

However, tunnel-etched foils not only have less surface area thancore-etched foils but are also quite brittle and tend to break easily,particularly when rolling or winding the foils to form cylindricalcapacitors. Accordingly, there remains a need to develop more durablefoil structures.

SUMMARY OF THE INVENTION

To address these and other needs, the present inventors devised a newfoil structure which combines the durability of core-etched foils withthe electrolyte-flow advantages of tunnel-etched foils. In addition todevising methods for making the new foil structure, the inventorsapplied the new foil structure in novel ways to build new capacitor foilassemblies and new capacitors in cylindrical and flat configurations,for example. Ultimately, these advances allow construction of smallermedical devices, such as implantable defibrillators.

Specifically, one embodiment of the new foil structure is a foil havingone or more holes or perforations and one or more cavities with a depthless than the foil thickness. In an exemplary embodiment, eachperforation and cavity has a cross-sectional area, with the perforationshaving a larger, for example, 2 to 100 times larger, averagecross-sectional area than the cavities. One method of making the newfoil structure includes perforating a foil and forming cavities into oneor both of its surfaces. Other methods form the cavities beforeperforating the foil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged perspective view of an exemplary foil structure 8that embodies the invention;

FIG. 2 is a perspective view of an exemplary cylindrical electrolyticcapacitor 10 which incorporates the foil structure of FIG. 1;

FIG. 3 is a cross-sectional view of an exemplary electrolytic capacitor10 which incorporates the foil structure of FIG. 1;

FIG. 4 is a cross-sectional view of a layered capacitive assembly 21which forms an active element 20 of capacitor 10 and which incorporatesthe FIG. 1 foil structure;

FIGS. 5A–5C are perspective views of other capacitor configurations thatincorporate the FIG. 1 foil structure;

FIG. 6 is a cross-sectional view of a symmetric capacitive assembly 60which incorporates the FIG. 1 foil structure and which is particularlysuited for flat capacitor configurations; and

FIG. 7 is a block diagram of generic implantable defibrillator 70including a capacitor that incorporates the FIG. 1 foil structure.

DETAILED DESCRIPTION

The following detailed description, which references and incorporatesFIGS. 1–7, describes and illustrates one or more exemplary embodimentsof the invention, specifically a new foil structure and method ofmanufacture, several new foil assemblies, new capacitors incorporatingthe foil structure and foil assemblies, and an implantable defibrillatorincorporating one or more of the new capacitors. These embodiments,offered not to limit but only to exemplify and teach, are shown anddescribed in sufficient detail to enable those skilled in the art toimplement or practice the invention. Thus, where appropriate to avoidobscuring the invention, the description may omit certain informationknown to those of skill in the art.

Exemplary Foil Structure and Methods of Manufacture

FIG. 1 shows an enlarged perspective view of a foil structure 8 whichthe inventors call a “perforated-core-etched” foil. Foil structure 8 canbe made of aluminum, tantalum, hafnium, niobium, titanium, zirconium,and combinations of these metals. However, the invention is not limitedto any particular foil composition or class of foil compositions.

Foil structure 8 includes opposing surfaces 8 a and 8 b which define anaverage foil thickness 8 t and a set of perforations 8 p which extendthrough foil structure 8 from surface 8 a to surface 8 b. Surfaces 8 aand 8 b include respective sets of surface cavities (or depressions) 9 aand 9 b, which have generally cylindrical, conical, or hemisphericalshapes. However, the invention is not limited to any particular cavityform, class of cavity forms, or combination of cavity forms. Surfacecavities 9 a have an average maximum depth Da which is less thanthickness 8 t, and surface cavities 9 b having an average maximum depthDb which is also less than thickness 8 t. As FIG. 1 shows, depths Da andDb are measured along dimensions generally perpendicular to respectivesurfaces 8 a and 8 b. Cavities 9 a and 9 b also have respective averagemaximum cross-sectional areas Sa and Sb (which are not shown in thefigure.) Cross-sectional area is measured in a plane substantiallyparallel to one of surfaces 8 a and 8 b.

In the exemplary embodiment, average maximum depths Da and depths Db areapproximately equal to one third or one quarter of thickness 8 t, andcross-sectional areas Sa and Sb are substantially equal and rangeinclusively between about 0.16 and 0.36 square-microns. However, otherembodiments use different equal and unequal depths Da and Db anddifferent and unequal cross-sectional areas Sa and Sb.

More generally, the exemplary embodiment adheres to the constraint thatthe sum of average maximum depths Da and Db is less than thickness 8 t.Adherence to this constraint ensures that a significant number ofcavities 9 a are separated from a significant number of cavities 9 b bya solid region of foil material. These regions of solid material notonly provide foil structure 8 with greater structural integrity but alsogreater surface area than conventional tunnel-etched foils. However, insome embodiments of the invention, one or more of cavities 9 a intersectone or more of cavities 9 b, thereby forming openings through the foil.The number of these intersections and resultant openings can beregulated through selection of appropriate cavity formation techniquesand cavity depths.

In addition to surface cavities 9 a and 9 b, FIG. 1 shows that foilstructure 8 includes a set of one or more perforations (or holes) 8 p.Perforations 8 p have an average maximum cross-sectional area Spmeasured in a plane substantially parallel to one of surfaces 8 a and 8b. Although perforations 8 p have a generally circular cross-section inthe exemplary embodiment, other embodiments use perforations withelliptical, triangular, square, or rectangular cross-sections. Thus, theinvention is not limited to any particular shape or class of shapes. Thelayout or arrangement of perforations 8 p takes any number of forms,including for example, a random distribution and a specific pattern witheach perforation having a predetermined position relative to otherperforations. The number of perforations per unit area is chosen tooptimize relevant criteria, such as capacitor electrical performance orfoil structural properties.

In the exemplary embodiment, average maximum cross-sectional area Sp ofperforations 8 p is larger than average maximum cross-sectional areas Saand Sb of cavities 9 a and 9 b. More precisely, area Sp in the exemplaryembodiment ranges between about 500 square-microns and 32square-millimeters. In other embodiments, area Sp ranges between 2–50,10–75, 25–100, or 2–100 times larger than surface areas Sa and Sb.Additionally, the exemplary embodiment provides a total perforation area(number of perforations times average maximum cross-sectional area Sp)which is no more than about 20 percent of the foil surface area.

The inventors have devised a number of ways of making foil structure 8.For example, one method initially core-etches a foil using conventionaletching techniques to form cavities 9 a and 9 b and then perforates thecore-etched foil. Another method entails initially perforating a foil toform perforations 8 p and then etching the perforated foil to formcavities 9 a and 9 b. (For more details on a conventional etchingtechnology, see, for example, U.S. Pat. No. 4,395,305 to Whitman, whichis entitled Chemical Etching of Aluminum Capacitor Foil and incorporatedherein by reference.) Perforations 8 p can be formed using lasers,chemical etchants, or mechanical dies, for example. Conceptually,cavities 9 a and 9 b could also be formed using lasers. Thus, theinvention is not limited to any particular technique or combination oftechniques for forming perforations 8 p and cavities 9 a and 9 b.

In one embodiment of the invention, further processing of the foils,particularly those intended for electrolytic capacitors, entailsapplying an insulative, or dielectric, coating to one or both sides ofthe foils. Examples of suitable coatings include metallic oxides such asaluminum or tantalum oxide.

Exemplary Foil Assemblies Incorporating the New Foil Structure

Foil structure 8 can be combined with other foils structures to formvarious electrically and/or mechanically advantageous foil assemblies.Many of these assemblies are particularly useful as multiple anodesstructures in flat, semi-cylindrical, and cylindrical capacitors.

In particular, the inventors devised several foil assemblies thatcombine foil structure 8 with core-etched and tunnel-etched foils. Forexample, one foil assembly stacks two or three foils incorporating foilstructure 8 to form a dual- or triple-foil assembly which can serve as adual or triple anode. Another foil assembly stacks a core-etched foilbetween two foils incorporating foil structure 8. Table 1 describesthese and several other foil assemblies.

TABLE 1 Foil Assembly No. Structure 1 PP 2 PPP 3 PCP 4 PPCPP 5 PTP 6PTPT 7 TPT 8 PTCTP

In the table, P denotes a perforated foil similar to foil structure 8; Cdenotes a core-etched foil; and T denotes a tunnel-etched foil. Thus,for example, foil assembly 7 comprises a foil similar to foil structure8 between two tunnel-etched foils. Other novel assemblies result fromcombining two or more of these assemblies. For instance, combining twoassembly Is yields a PPPP structure, and combining assemblies 2 and 3yields a PPPPCP structure. Additionally, still other novel assembliesresult from inserting insulators and electrolyte-impregnated substrates,such as paper, between adjacent foils of an assembly.

Exemplary Capacitor Incorporating the New Foil Structure

FIG. 2 shows a perspective view of an exemplary electrolytic capacitor10 which incorporates one or more foils incorporating foil structure 8or one or more of the foil assemblies described above. In addition toincorporating these novel foils and foil assemblies, capacitor 10embodies many novel space-saving features. These features and theiradvantages are addressed in a co-pending U.S. patent application Ser.No. 10/165,848 which is entitled Smaller Electrolytic Capacitors forImplantable Defibrillator, and which was filed on the same day as thepresent application. This application is incorporated herein byreference.

More specifically, FIG. 2 shows that capacitor 10 has a diameter 10 d ofabout 14.5 millimeters and a total height 10 h of about 30 millimeters,thereby occupying a total volume of about five cubic-centimeters.Capacitor 10 also includes a cylindrical aluminum case 12, a header (orlid) 14, and two aluminum terminals 16 and 18. Two rivets 15 and 17fasten terminals 16 and 18 to header 14. Case 12, which houses an activeelement 20 (not visible in this view), includes a circumferentialseating groove 12 a and a rolled lip 12 b, both of which secure header14 to case 12.

FIG. 3, a cross-sectional view taken along line 3—3 in FIG. 2, showsthat case 12 has a thickness 12 t and groove 12 a is spaced a distance12 d from lip 12 b. Thickness 12 t is about 0.010 inches, and distance12 d is about 0.145 inches. Additionally, groove 12 a has a radius ofabout 0.035 inches, and lip 12 b, which is formed by rolling over thetop edge of case 12, has a radius of about 0.015 inches. (Someembodiments compress or flatten groove 12 a to reduce capacitor heightand volume.) Header 14, which comprises a rubber layer 14 a and aphenolic-resin layer 14 b, has a total thickness 14 t of about twomillimeters.

FIG. 3 also shows that capacitor 10 includes an active element 20 woundaround mandrel region 28 and two pairs of insulative inserts 30 a-30 band 32 a-32 b respectively positioned adjacent the top and bottom ofactive element 20. Mandrel region 28 has an exemplary width or diameter28 w of about 2.5 millimeters. And, insulative inserts 30 a–30 b and 32a–32 b comprise respective pairs of paper disks, with each disk having athickness of one one-thousandth of an inch and a diameter of about 14millimeters The insulative inserts ensure electrical isolation ofconductive portions of active element 20 from anode tab 25 and rivets 15and 17 and from the bottom interior surface of case 12. (As analternative to insulative inserts, other embodiments enclosesubstantially all of active element 20 within an insulative bag.) Forclarity, FIG. 3 omits a 1.125-inch-wide plastic insulative sheath thatsurrounds the vertical surfaces of active element 20.

Active element 20 comprises about 19 turns of a layered capacitiveassembly 21. As the cross-section in FIG. 4 shows, capacitive assembly21 includes a cathode 22, an anode structure 24, and fourelectrolyte-impregnated separators 26 a, 26 b, 26 c, and 26 d. Cathode22 and anode 24 each have a width (or height) 22 w. In this exemplaryembodiment, cathode 22 and the one or more constituents of anodestructure 24 are about 24 millimeters wide and 100 microns thick.Cathode 22 is about 422 millimeters long, and anode structure 24 isabout 410 millimeters long.

Anode structure 24 can assume a variety of novel forms, the simplestbeing a single foil member incorporating foil structure 8 of FIG. 1.Some embodiments provide anode structure 24 with one or more of thenovel foil assemblies described using Table 1.

Although not shown in FIG. 4, the exemplary embodiment connects anodestructure 24 to one anode tab regardless of the number of foilsconstituting the anode structure. (FIG. 3 shows an exemplary aluminumanode tab 25.) Other embodiments, however, provide individual anode tabsfor each anode members, with the tabs connected together to form a jointor composite anode tab. For more details on these or other types of tabsincorporated in other embodiments of the invention, see co-pending U.S.patent applications Ser. Nos. 09/063,692 and 09/076,023 which arerespectively entitled Electrolytic Capacitor and Multi-Anodic Attachmentand Wound Multi-Anode Electrolytic Capacitor with Offset Anodes andwhich are incorporated herein by reference.

Anode tab 25, shown in FIG. 3, is ultrasonically welded to rivet 15 andthus electrically connected to terminal 16. The exemplary embodimentfolds anode tab 25 over itself; however, other embodiments omit thisfold to reduce the space between header 14 and the top of active element20. Though not visible in FIG. 3 or 4, cathode 22 includes a cathode tabwhich is similarly connected via rivet 17 to terminal 18.

In addition to cathode 22 and anode 24, FIG. 4 shows that capacitiveassembly 21 includes thin electrolyte-impregnated separators 26,specifically 26 a, 26 b, 26 c, and 26 d. In the exemplary embodiment,separators 26 a–26 d each consists of kraft paper impregnated with anelectrolyte, such as an ethylene-glycol base combined withpolyphosphates or ammonium pentaborate, and each has a thickness lessthan 0.001 inches. More specifically, the exemplary embodiment uses oneor more papers of the following thicknesses: 0.000787, 0.0005 inches,and 0.00025 inches, with thicker papers preferably placed nearer thecenter of the active element to withstand the greater tensile stressthat interior separators experience during winding.

Additionally, each of separators 26 a–26 d has a width 26 w which isless than four millimeters wider than cathode 22 and anode 24 to provideend margins 27 a and 27 b. In the exemplary embodiment, width 26 w isabout 27 millimeters, or three millimeters wider than cathode 22 andanode 24, to provide end margins 27 a and 27 b of about 1.5 millimeters.Other embodiments of the invention provide at least one end margins ofabout 1.75, 1.25, 1, 0.75, 0.5, 0.25, and even 0.0 millimeters.

Although the exemplary capacitor 10 has a wound or cylindricalconfiguration, the invention is not limited to any particular type orcategory of configurations. For example, FIGS. 5A, 5B, and 5C show othercapacitor configurations encompassed by the invention. Morespecifically, FIG. 5A shows a semi-cylindrical (or “D”) capacitor; FIG.5B shows an asymmetric semi-cylindrical capacitor; and FIG. 5C shows aflat (or more precisely a rectangular parallelepiped) capacitor. FIG. 6show a cross-sectional view of a capacitive assembly 60 particularlyuseful for flat capacitors such as the one shown in FIG. 5C.

In particular, capacitive assembly 60 includes an anode structure 62between two cathode foils 64 a and 64 b. Electrolyte-impregnatedseparators 63 a and 63 b lie respectively between anode structure 62 andcathode foils 64 a and 64 b. In the exemplary embodiment separators 63 aand 63 b each comprise two or more layers of kraft paper of thicknessessimilar to separators 26 of FIG. 4. Anode structure 62 comprises one ormore of the foil assemblies identified in Table 1.

Exemplary Embodiment of Implantable Defibrillator

FIG. 7 shows one of the many applications for exemplary capacitor 10: ageneric implantable defibrillator 70. More specifically, defibrillator70 includes a lead system 72, which after implantation electricallycontacts strategic portions of a patient's heart, a monitoring circuit74 for monitoring heart activity through one or more of the leads oflead system 72, and a therapy circuit 76 which delivers electricalenergy through lead system 72 to the patient's heart. Therapy circuit 76includes an energy storage component 76 a which incorporates at leastone capacitor having one or more of the novel features of capacitor 10.Defibrillator 70 operates according to well known and understoodprinciples.

In addition to implantable defibrillators, the innovations of capacitor10 can be incorporated into other cardiac rhythm management systems,such as heart pacers, combination pacer-defibrillators, anddrug-delivery devices for diagnosing or treating cardiac arrhythmias.They can be incorporated also into non-medical applications, forexample, photographic flash equipment. Indeed, the innovations ofcapacitor 10 are pertinent to any application where small, high energy,low equivalent-series-resistance (ERS) capacitors are desirable.

CONCLUSION

In furtherance of the art, the inventors devised a new foil structurewhich combines the durability of core-etched foils with the electrolyteflow advantages of tunnel-etched foils. In addition to devising methodsfor making the new foil structure, the inventors applied the new foilstructure to build new capacitors and implantable defibrillators.

The embodiments described above are intended only to illustrate andteach one or more ways of practicing or implementing the presentinvention, not to restrict its breadth or scope. The actual scope of theinvention, which embraces all ways of practicing or implementing theconcepts and principles of the invention, is defined only by thefollowing claims and their equivalents.

1. A cardiac rhythm management system comprising: a therapy circuit fordelivering therapy to a heart of a patient, wherein the therapy circuitincludes one or more capacitors which include a foil that comprises:first and second opposing surfaces which define a foil thickness, withat least one of the opposing surfaces having a plurality of cavitieseach having a maximum depth less than the foil thickness, wherein theaverage depth of the cavities is about one-third or one-fourth of thefoil thickness; and a plurality of perforations through the foil.
 2. Acardiac rhythm management system comprising: a therapy circuit fordelivering therapy to a heart of a patient, wherein the therapy circuitincludes one or more capacitors, with each capacitor including at leastone foil that comprises: first and second opposing sides each having oneor more of cavities having an average depth less than one-half the foilthickness; and three or more perforations through the at least one foil,wherein the average cross-sectional area of the perforations is about 2to 100 times larger than the average cross-sectional area of thecavities, wherein the at least one foil comprises at least one ofaluminum, tantalum, hafnium, niobium, titanium, and zirconium, andwherein each of the first and second opposing sides each having aninsulative coating.
 3. The system of claim 2 wherein each perforationhas a generally circular cross-section.
 4. The system of claim 2 whereinat least a portion of the one foil is rolled.
 5. The system of claim 2wherein the one foil is part of an anode structure.
 6. The system ofclaim 2 wherein the average depth of the cavities is about one-third orone-fourth of the foil thickness.
 7. A cardiac rhythm management systemcomprising: one or more leads; a monitoring circuit for monitoring heartactivity of a patient through one or more of the leads; and a therapycircuit coupled to the monitoring circuit, wherein the therapy circuitincludes one or more capacitors which include a foil that comprises:first and second opposing surfaces which define a foil thickness, withat least the first opposing surface having a plurality of cavities eachhaving a maximum depth less than the foil thickness and the cavitieshaving an average cross-sectional area; and a plurality of at leastthree perforations through the foil, with the perforations having anassociated average maximum cross-sectional area that is greater than theaverage cross-sectional area of the cavities, and the perforationshaving a total cross-sectional area that is less than 20 percent of thefoil surface area.
 8. The system of claim 7, wherein the perforationshave an associated average maximum cross-sectional area between 500square-microns and 32 square-millimeters, inclusive.
 9. The system ofclaim 7, wherein the cavities have an average depth less than one-halfthe foil thickness.
 10. The system of claim 7, wherein the average depthof the cavities is about one-third or one-fourth of the foil thickness.11. The system of claim 7, wherein the perforations are randomlydistributed.
 12. The system of claim 7, wherein the averagecross-sectional area of the perforations is about 2 to 100 times largerthan the average cross-sectional area of the cavities.
 13. The system ofclaim 7, wherein each perforations has a generally circularcross-section.
 14. The system of claim 7, wherein the foil comprisestitanium.
 15. The system of claim 7, wherein one or more of the cavitiesdefine a perimeter which circumscribes one or more of the perforationsand wherein the foil is at least partly rolled.
 16. A cardiac rhythmmanagement system comprising: one or more leads for sensing electricalsignals of a patient; a monitoring circuit coupled to the one or moreleads for monitoring heart activity of the patient; and a therapycircuit coupled to the one or more of the leads, wherein the therapycircuit includes one or more capacitors which include two or more foilsstacked according to one or more of the following foil sequences: PP,PPP, PCP, PPCPP, PTP, PTPT, TPT, PTCTP, wherein each C denotes acore-etched foil; each T denotes a tunnel-etched foil; and each Pdenotes a foil having a foil thickness and comprising a plurality ofperforations and a plurality of cavities, with each cavity having adepth less than the foil thickness.
 17. The system of claim 16: whereineach P foil has first and second opposing sides, with one or more of thecavities on the first side and one or more of the cavities on the secondside and wherein the average depth of the cavities is less than one-halfthe foil thickness; or wherein the perforations have an averagecross-sectional area larger than an average cross-sectional area of thecavities; or wherein the average cross-sectional area of theperforations is about 2 to 100 times larger than an average maximumcross-sectional area of the cavities; or wherein one or more the foilscomprises titanium.
 18. A cardiac rhythm management system comprising:one or more leads; a monitoring circuit for monitoring heart activity ofa patient through one or more of the leads; and a therapy circuitcoupled to the one or more leads, wherein the therapy circuit includesone or more capacitors which include: at least oneperforated-core-etched foil adjacent at least one non-perforatedcore-etched foil; or at least one perforated-core-etched foil, at leastone non-perforated core-etched foil and at least one tunnel-etched foil.19. A cardiac rhythm management system comprising: one or more leads; amonitoring circuit for monitoring heart activity of the patient throughone or more of the leads; and a therapy circuit coupled to themonitoring circuit, wherein the therapy circuit includes one or morecapacitors which include: at least one perforated-core-etched foil; orat least one perforated-core-etched foil adjacent at least onecore-etched foil; or at least one perforated-core-etched foil, at leastone core-etched foil, and at least one tunnel-etched foil; wherein eachperforated core-etched foil comprises: first and second opposingsurfaces which define a foil thickness, with the first opposing surfacehaving one or more cavities each having a maximum depth less than thefoil thickness and the second opposing surface having one or morecavities each having a maximum depth less than the foil thickness; andthree or more perforations through the foil.
 20. The capacitor of claim19, wherein the cavities of the first surface have a first average depthand the cavities of the second surface have a second average depth, withthe first and second average depths being less than one-half the foilthickness.
 21. A cardiac rhythm management system comprising: one ormore leads; a monitoring circuit for monitoring heart activity of thepatient through one or more of the leads; and a therapy circuit coupledto the monitoring circuit, wherein the therapy circuit includes one ormore capacitors which include a capacitor comprising one or moretunnel-etched foils between two or more perforated-core-etched foils.22. The cardiac rhythm management system of claim 21, wherein eachperforated core-etched foil comprises: first and second opposingsurfaces which define a foil thickness, with the first opposing surfacehaving one or more cavities each having a maximum depth less than thefoil thickness and the second opposing surface having one or morecavities each having a maximum depth less than the foil thickness; andthree or more perforations through the foil.